Morphologic description of the Punta Cola rock avalanche and associated minor rockslides caused by the 21 April 2007 Aysén earthquake (Patagonia, southern Chile)

Abstract

The 21 April 2007 Aysén earthquake triggered hundreds of landslides in the epicentral area, including several large-volume rock avalanches that caused tsunamis in the Aysén fjord. This study is part of a larger project aiming to better understand the conditioning factors controlling size and distribution of earthquake-triggered landslides. The focus is set on the largest rockslide that occurred at Punta Cola. Using a high-resolution digital surface model created from terrestrial laser scanning, a detailed geomorphologic map of the rockslide scars and the rock avalanche deposits on land was made. This map permitted to establish the sequence of events: the failure of the rockslides main compartment led to a rock avalanche that crossed the valley and ran up on the opposite valley flank before turning downstream and impacting the fjord and creating a tsunami. Afterwards, another rockslide compartment failed and piled up 80 m thick deposits in the valley. Seven minor rockslides occurred in the Punta Cola valley after the main rock avalanche. The volumes of rockslides and associated deposits were estimated using the sloping local base level technique that allows computing the possible pre-rockslide topography and the bedrock-deposits interface. These volume computations give 22.4 Mm³ for the Punta Cola rockslide, which is significantly more than previous estimates (12 Mm³). The on-shore deposits of the main rock avalanche have a volume of 13.7 Mm³ and approximately 14.4 Mm³ are deposited in the fjord. This study highlights the additional information on rockslides and rock avalanches that can be obtained from high-resolution 3D data and mapping.